This invention relates to implantable medical devices (IMDs) and their various components, including flat electrolytic capacitors for same, and methods of making and using same, particularly such capacitors formed of a plurality of stacked capacitor layers each having anode layers formed of one or a plurality of partially through-etched and through-hole punctured anode sheets.
As described in commonly assigned U.S. Pat. No. 6,006,133, a wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high energy cardioversion and/or defibrillation shocks to a patient""s heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Current ICDs typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier developed automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Energy, volume, thickness and mass are critical features in the design of ICD implantable pulse generators (IPGs) that are coupled to the ICD leads. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrillation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is desirable to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Doing so is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality to the ICD IPG. It is also desirable to provide such ICD IPGs at low cost while retaining the highest level of performance. At the same time, reliability of the capacitors cannot be compromised.
Various types of flat and spiral-wound capacitors are known in the art, some examples of which are described as follows and/or may be found in the patents listed in Table 1 of the above-referenced, commonly assigned, ""133 patent. Typically, an electrolytic capacitor is formed with a capacitor case enclosing an etched aluminum anode layer (or xe2x80x9celectrodexe2x80x9d), an aluminum cathode layer (or xe2x80x9celectrodexe2x80x9d), and a Kraft paper or fabric gauze spacer or separator impregnated with a solvent based liquid electrolyte interposed therebetween. A layer of aluminum oxide that functions as a dielectric layer is formed on the etched aluminum anode, preferably during passage of electrical current through the anode layer. The electrolyte comprises an ion producing salt that is dissolved in a solvent and provides ionic electrical conductivity between the cathode and the aluminum oxide dielectric layer. The energy of the capacitor is stored in the electromagnetic field generated by opposing electrical charges separated by the aluminum oxide layer disposed on the surface of the anode layer and is proportional to the surface area of the etched aluminum anode layer. Thus, to minimize the overall volume of the capacitor one must maximize anode surface area per unit volume without increasing the capacitor""s overall (i.e., external) dimensions. The separator material, anode and cathode layer terminals, internal packaging, electrical interconnections, and alignment features and cathode material further increase the thickness and volume of a capacitor. Consequently, these and other components in a capacitor and the desired capacitance limit the extent to which its physical dimensions may be reduced.
Some ICD IPGs employ commercial photoflash capacitors similar to those described by Troup in xe2x80x9cImplantable Cardioverters and Defibrillators,xe2x80x9d Current Problems in Cardiology, Volume XIV, Number 12, December 1989, Year Book Medical Publishers, Chicago, and as described in U.S. Pat. No. 4,254,775. The electrodes or anode and cathodes are wound into anode and cathode layers separated by separator layers of the spiral. Most commercial photoflash capacitors contain a core of separator paper intended to prevent brittle, highly etched aluminum anode foils from fracturing during winding of the anode, cathode and separator layers into a coiled configuration. The cylindrical shape and paper core of commercial photoflash capacitors limits the volumetric packaging efficiency and thickness of an ICD IPG housing made using same.
Flat electrolytic capacitors have also been disclosed in the prior art for general applications as well as for use in ICDs. More recently developed ICD IPGs employ one or more flat high voltage capacitor to overcome some of the packaging and volume disadvantages associated with cylindrical photoflash capacitors. For example, U.S. Pat. No. 5,131,388 discloses a flat capacitor having a plurality of stacked capacitor layers each comprising an xe2x80x9celectrode stack sub-assemblyxe2x80x9d. Each capacitor layer contains one or more anode sheet forming an anode layer having an anode tab, a cathode sheet or layer having a cathode tab and a separator for separating the anode layer from the cathode layer. In the ""388 patent, the electrode stack assembly of stacked capacitor layers is encased within a non-conductive, polymer envelope that is sealed at its seams and fitted into a chamber of a conductive metal, capacitor case or into a compartment of the ICD IPG housing, and electrical connections with the capacitor anode(s) and cathode(s) are made through feedthroughs extending through the case or compartment wall. The tabs of the anode layers and the cathode layers of all of the capacitor layers of the stack are electrically connected in parallel to form a single capacitor or grouped to form a plurality of capacitors. The aluminum anode layer tabs are gathered together and electrically connected to a feedthrough pin of an anode feedthrough extending through the case or compartment wall. The aluminum cathode layer tabs are gathered together and electrically connected to a feedthrough pin of a cathode feedthrough extending through the case or compartment wall or connected to the electrically conductive capacitor case wall.
Many improvements in the design of flat aluminum electrolytic capacitors for use in ICD IPGs have been disclosed, e.g., those improvements described in xe2x80x9cHigh Energy Density Capacitors for Implantable Defibrillatorsxe2x80x9d presented by P. Lunsmann and D. MacFarlane at CARTS 96: 16th Capacitor and Resistor Technology Symposium, Mar. 11-15 1996, and at CARTS-EUROPE 96: 10th European Passive Components Symposium, Oct. 7-11 1996, pp. 35-39. Further features of flat electrolytic capacitors for use in ICD IPGs are disclosed in U.S. Pat. Nos. 4,942,501; 5,086,374; 5,146,391; 5,153,820; 5,562,801; 5,584,890; 5,628,801; and 5,748,439, all issued to MacFarlane et al.
Typically, the anode layer of each capacitor layer is formed using a single highly etched anode sheet or a plurality of such anode sheets cut from a highly etched metallic foil. Highly etched aluminum foil has a microscopically contoured, etched surface with a high concentration of pores extending part way through the anode foil along with tunnels extending all the way through the anode foil (through-etched or tunnel-etched) or only with a high concentration of pores extending part way through the anode foil (nonthrough-etched). In either case, such a through-etched or nonthrough-etched anode sheet cut from such highly etched foil exhibit a total surface area much greater than its nominal (length times width) surface area. A surface area coefficient, the ratio of the microscopic true surface area to the macroscopic nominal area, may be as high as 100:1 which advantageously increases capacitance. Through-etched or tunnel-etched anode sheets exhibit a somewhat lower ratio due to the absence of a web or barrier surface closing the tunnel as in nonthrough-etched anode sheets.
After the aluminum foil is etched, the aluminum oxide layer on the etched surface is formed by applying voltage to the foil through an electrolyte such as boric acid or citric acid and water or other solutions familiar to those skilled in the state of the art. Typically, individual anode sheets are punched, stamped or otherwise cut out of the foil in a shape to conform to the capacitor package following formation of the aluminum oxide on the foil. The cut edges around the periphery of the anode sheets are carefully cleaned to remove particulates of anode material that can get caught between the capacitor layers in the electrode stack assembly resulting in a high leakage current or capacitor failure. Anode layers are formed of either a single anode sheet or multiple anode sheets. Capacitor layers are assembled by stacking the anode layer, separator layers, and cathode layer together, and electrode stack assemblies are assembled by stacking a plurality of capacitor layers together, separated by separator layers. The cut edges of the anode and cathode layers and any other exposed aluminum are then reformed in the capacitor during the aging process to reduce leakage current.
Non-through-etched anode sheets are used when only one anode sheet is employed as the anode layer. In order to increase capacitance (and energy density), multiple anode sheets are stacked together to form the multiple sheet anode layer as described above. Through-etched or tunnel-etched anode sheets need to be used in such multiple sheet anode layers to ensure that electrolyte is distributed over all of the aluminum oxide layers of the sandwiched inner anode sheets and to provide a path for ionic communication. But, then the gain in surface area is not as high as that which can be achieved with a like number of stacked nonthrough-etched anode sheets that have a remaining solid section in their center.
For example, the ""890 patent discloses the use of an anode layer formed of three anode sheets comprising a highly etched sheet with a solid core in the center and two tunnel-etched anode sheets sandwiching the center sheet. This arrangement is intended to allow the electrolyte and thus the conducting ions to reach the whole surface area of the anode layer, even pores which originate on the inner layer of the foil, yet at the same time the ions are not able to penetrate all the way through the anode layer. More tunnel etched anode sheets can be used in the sandwiched anode layer depending on the desired electrical performance.
Electrical performance of such electrolytic capacitors is effected by the surface area of the anode and cathode layers and also by the resistance associated with the electrolytic capacitor itself, called the equivalent series resistance (ESR). The ESR is a xe2x80x9chypotheticalxe2x80x9d series resistance that represents all energy losses of an electrolytic capacitor regardless of source. The ESR results in a longer charge time (or larger build factor) and a lower discharge efficiency. Therefore, it is desirable to reduce the ESR to a minimum.
One of the elements of the ESR is the solution resistance inside the pores or tunnels of the anode sheets formed during the electrochemical etching to increase the anode surface area capacitance. The size and depth of a pore and the size of a tunnel through the anode sheet depend on the etching process as well as the oxide formation process. To minimize ESR, the tunnels should be big enough for oxide to grow and long enough for ions to migrate through the anode sheets of the anode layer. In other words, the ideal anode sheet should have pores or tunnels that penetrate through the sheet thickness and are large enough for the electrolyte to flow therethrough. In reality, pores and tunnels vary in size. Narrow tunnels can retard ion transfer, and the pores that are not through-etched tunnels block the paths for ionic migration.
However, as noted above, through-etched tunnels decrease anode layer surface area and reduce the capacitance of a capacitor layer formed with such anode layers in comparison to the capacitance of an equivalent capacitor layer formed using a nonthrough-etched anode layer It is difficult to control the etching parameters to ensure that a minimum number of tunnels having a sufficiently large cross-section to minimize ESR are created so as to maximize capacitance.
High surface area is created during the Electrochemical etching process by dissolving aluminum and forming tunnels or holes. However, the electrochemical tunnel etching is a xe2x80x9crandomxe2x80x9d process, resulting in uncontrollable tunnel site distribution and various tunnel sizes and lengths. As a result, the capacitance of commercial aluminum foils is much lower than that of the ideal foil having site-controllable tunnels with the same size and length.
In order to use multiple anode foil sheets in a xe2x80x9cmultiple anode sheet configurationxe2x80x9d some of the tunnels are through-etched, allowing electrolyte communication on both sides of the foil. In reality, the density of through-etched tunnels is limited in practiced because a high density of through-etched tunnels causes the aluminum foil or sheets cut from the foil to be very brittle. The brittle foil or sheets are very difficult to handle in further processes of forming electrolytic capacitors. Therefore, in practice, the ESR is not sufficiently reduced because of the limited number, size and density of through-etched tunnels extending between the foil or sheet sides.
It is desirable to overcome these problems with providing ionic communication through anode sheets to minimize ESR and maximize surface area.
The present invention provides for anode layers of electrolytic capacitors that minimize ESR and maximize surface area wherein such capacitors are formed of one or a plurality of stacked capacitor layers each having anode layers formed of one or a plurality of partially through-etched and through-hole punctured anode sheets.
This invention provides paths for electrolyte transfer by forming small through-holes through xe2x80x9cpartiallyxe2x80x9d through-etched anodes in order that the ESR is reduced and there are more paths for the ions to migrate. The number and size of these through-holes are chosen to reduce the ESR to a minimum while not unnecessarily reducing surface area. In general a minimal number and size of through-hole will be chosen so that the finished capacitor still meets the application requirements. The through-holes need not be round, but that is a convenient shape to use.
In one embodiment employing multiple anode sheets sandwiched together forming an anode layer, the innermost anode sheet is nonthrough etched and not punctured to form a barrier to ion migration or communication through the innermost anode sheet, whereas the outer anode sheets are punctured to enable ion migration and electrolyte distribution to all anode sheet surfaces.
In one embodiment, an exemplary electrolytic capacitor formed in accordance with the present invention comprises an electrode stack assembly and electrolyte located within the interior case chamber of a hermetically sealed capacitor case. The electrode stack assembly comprises a plurality of capacitor layers stacked in registration upon one another, each capacitor layer comprising a cathode layer having a cathode tab, an anode layer comprising at least one anode sheet having an anode tab, and a separator layer located between adjacent anode and cathode layers, whereby all adjacent cathode layers and anode layers of the stack are electrically insulated from one another by a separator layer. Anode terminal means extend through the capacitor case side wall for electrically connecting a plurality of the anode tabs to one another and providing an anode connection terminal at the exterior of the case. Cathode terminal means extend through or to an encapsulation area of the capacitor case side wall for electrically connecting a plurality of the cathode tabs to one another and providing a cathode connection terminal at the exterior of the case. A connector assembly is electrically attached to the anode connection terminal for making electrical connection with the anode tabs and to the cathode connection terminal for making electrical connection with the cathode tabs.
The present invention provides a more controllable method for forming through holes in partially through-hole etched aluminum sheets or foils used to form anode layers. The ESR is thereby reduced while aluminum surface area and capacitance is maintained.
The puncturing method generates more tunnels in aluminum foils used in making aluminum anode sheets for anode layers. However, it also re removes the existing tunnels that are created during the electrochemical etching. Since the existing technology can not generate tunnels smaller than those created during the etching process, the net result is surface area reduction and thus capacitance loss. The degree of capacitance loss depends on the hole size and density. However, since more through-holes arc created, the ESR is lower.
Since the puncturing process reduces foil capacitance, it should not be overdone. The hole size and density need to be controlled such that the ESR is lowered but the capacitance is not reduced too much.